The present invention relates to the control of printing heads, and particularly for impact printers, such as dot matrix printers operating at very high data rates, to insure uniformly good quality printing.
A fairly common type of printer is a dot matrix printer. This type of printer involves a plurality of print wires or styli which are arranged in one or more vertical lines and are maintained in a spaced-apart arrangement in a print head. The head is supported on a carriage which in turn is caused to traverse a line of movement across a record medium. A common type of dot pattern involves a 5.times.7 matrix. As the carriage shifts the print head through the successive columns across a line of movement on the record medium, a 5.times.7 dot pattern of alphanumeric characters or symbols is produced on the record medium by selectively displacing or extending the individual print wires in their successive column positions to impact the record medium through an inked ribbon.
Each print wire or stylus typically has a drive circuit for controlling the operation thereof. Signals from an associated source such as a keyboard are fed to a matrix encoder which converts them to signals of the matrix format for controlling the print wire drive circuits. The drive circuits usually attempt to supply constant energy drive pulses, based upon the assumption that constant energy drive pulses always produce constant impact forces and uniform print intensity.
It has been found, however, that this assumption is incorrect when an electromagnetic print head element operates at or near a maximum repetition rate established by the dynamics of its spring-mass system where this rate exceeds the response capability of the electromagnetic system involved. More particularly, as the repetition rate at which the print wires are operated is increased beyond a predetermined threshold rate, a condition is arrived at wherein there is insufficient time between adjacent drive pulses to permit complete decay of the magnetic field in the print head. Thus, at the initiation of the next driving pulse, some magnetic energy from the previous pulse remains in the print head. Consequently, if equal energy is used for all driving pulses, this energy will be added to the residual energy left from the preceding pulse and will cause the print wires to print harder at printing rates approaching the maximum print rate than they do at lower printing rates which permit sufficient time for a complete decay of the magnetic energy of the print head between driving pulses.
This uneven printing which occurs when the print head is operating at or near its maximum designed rate presents a significant problem since it has been found that, depending on the data pattern, approximately 35 percent of the printed dots in a typical text require maximum rate response of the print head. In addition to uneven printing, the excessive forces resulting from overdriving of the fast-rate dots can result in ribbon damage as well as overheating and/or excessive wear of the print head. Furthermore, when rapidly printed dots are overdriven, the print wires may impact and try to return to their rest positions before the magnetic fields driving them start to decay. The residual magnetic field may oppose the return movement of the print wires, thereby limiting print rate and introducing undesirable effects, such as variation in dot spacing and skipped dots.
Nor it is practicable to reduce the energy of all of the drive pulses to a level which will produce the proper print intensity for high-rate pulses, since this energy level will be insufficient to produce adequate print intensity of pulses printed at lower rates. Therefore, a means for producing uniform print intensity at all print rates is clearly needed.
U.S. Pat. No. 3,866,533, which issued to R. L. Gilbert et al. on Feb. 18, 1975, discloses means for changing the width of drive pulses applied to the print hammers of a high-speed printer to compensate for variations in the source voltage and variation in the thickness of the record medium being imprinted.
U.S. Pat. No. 3,172,353, which issued to C. J. Helms on Mar. 9, 1965, discloses means for extending the length of the drive pulses beyond the time at which the print hammer strikes the record medium so that the print head magnetic field will oppose the print hammer rebound and thereby dissipate a portion of the rebound energy so as to minimize backstop wear.
Neither of these prior art patents discloses a matrix printer, but rather each discloses a printer of the type in which each print hammer prints a complete character. Therefore, the maximum repetitive impact rates in these prior art patents is inherently considerably less than that of a matrix printer wherein a plurality of repetitive print wire strokes are required to print most characters. The aforementioned prior art patents are therefore not concerned with uneven print intensity resulting from accommodating a wide range of print rates involving incomplete magnetic field decay between successive impulses.